5.9. /var/lock : Lock files

5.9.1. Purpose

Lock files should be stored within the
/var/lock directory structure.

Lock files for devices and other resources shared by multiple
applications, such as the serial device lock files that were
originally found in either /usr/spool/locks or
/usr/spool/uucp, must now be stored in
/var/lock. The naming convention which must be
used is "LCK.." followed by the base name of the device. For example,
to lock /dev/ttyS0 the file "LCK..ttyS0" would be created.
[43]

The format used for the contents of such lock files must be the
HDB UUCP lock file format. The HDB format is to store the process
identifier (PID) as a ten byte ASCII decimal number, with a trailing
newline. For example, if process 1230 holds a lock file, it would
contain the eleven characters: space, space, space, space, space,
space, one, two, three, zero, and newline.

[43] Then, anything wishing to use /dev/ttyS0
can read the lock file and act accordingly (all locks in
/var/lock should be world-readable).